T.M. Yue

Corrosion properties of laser surface melted NiTi shape memory alloy

A Nd-YAG laser was used to melt the surface of NiTi shape memory alloy in air and argon environment respectively. The laser surface melted (LSM) layer is free of porosity and crack. The corrosion resistance of the LSM specimen in 3%NaCl solution has been improved significantly and is attributed to the increased amount of TiO2 and Ti/Ni ratio on the surface.

Un-conventional machining of composite materials

Abstract Using un-conventional machining techniques in shaping advanced composites materials has generated a lot of interest. The fact is that, machining operations such as cutting and drilling, are difficult to preform on these materials with conventional tools and techniques because of their peculiar properties including anisotropy, low thermal conductivity and the abrasive nature of the reinforcing phases. Among the many un-conventional processing techniques laser and electrical discharge machining (EDM) have been proved to be effective tools in shaping some of these materials. This paper not only provides a systematic analysis on the machining efficiency that can be obtained by laser and EDM and the maximum depth-of-cut that can be generated by laser, the important issue of machining defects is also addressed. It is hoped that this study will be helpful to the further understanding of the machining mechanisms of Nd:YAG laser, excimer laser and electrical discharge method and thereby to improve the techniques in the processing of composite materials.

Squeeze casting of light alloys and their composites

Abstract Squeeze casting is the term used commonly to describe processes in which liquid metal is solidified under the action of a relatively high external pressure. The fundamentals of the process have been known for decades, the process having been used in Russia for more than 50 years. However, it is only recently that squeeze casting has been commercialised in the West to produce high-quality engineering components with, and without, reinforcements. This article reviews aspects of recent progress in the development of current squeeze-casting techniques, and addresses both the merits and limitations of the various processes. The relationships between the microstructure and mechanical properties of some squeeze-cast light alloys, as well as their metal-matrix composite (MMC) counterparts, are presented, and the distinguished advantage of being able to directly squeeze cast high-strength aerospace wrought alloys, thus replacing forgings by casting, is highlighted.

Modeling Young's Modulus of Polymer-layered Silicate Nanocomposites Using a Modified Halpin—Tsai Micromechanical Model

In this study nanocomposites consisting of an epoxy matrix filled with silicate clay particles are investigated. Recent and ongoing research has shown that dramatic enhancements can be achieved in stiffness and thermal properties in these nanocomposites with small amounts of particle concentration. The resulting properties of nanocomposites are intimately related to the microstructure achieved in processing these materials. However, the ideal situation of full exfoliation, dispersion, and orientation is not usually achieved. A more common case is the partial exfoliation and intercalation. The latter is a process whereby the polymer penetrates the interlayer spaces of clay particles, causing an increase in the layer spacing (d-spacing). The region consisting of a matrix with exfoliated clay nanolayers or platelets is analyzed by assuming a near uniform-dispersion and a random orientation. The properties of intercalated clusters of clay platelets are calculated by a rule of mixtures based on a parallel platelet system. The modified composite theory of Halpin—Tsai is applied to calculate the modulus of the nanocomposite as a function of the clay concentration for various parametric variations, including the exfoliation ratio, the particle/matrix stiffness ratio Ef/Em, the particle volume fraction f, and the particle aspect ratio L/t. The modified composite theory satisfactorily captures the stiffness behavior of the polymer/clay composites.

A study of ultrasonic-aided wire electrical discharge machining

Abstract An investigation has been made into the combined technology of ultrasonic and wire electrical discharge machining (W-EDM), in which the design of the vibrating device is described. The theory to describe the vibration modes of the wire under the action of ultrasonic has been established. Experimental results show that wire vibration induced by ultrasonic action has a significant effect on the overall performance of the W-EDM process. It was found that there exists an optimum relationship between the vibration amplitude of the wire and the discharge energy, by which the highest cutting rate and the best machined surface quality can be obtained. In addition, ultrasonic vibration reduced the residual tensile stress of the machined surface.

Microstructure and wear properties of laser surface-cladded Mo–WC MMC on AA6061 aluminum alloy

Abstract The creation of a wear-resistant surface of metal matrix composite on AA6061 aluminum alloy by laser surface melting of pre-pasted Mo–WC powder was successfully achieved using a 2-kW CW Nd-YAG laser. A high-quality coating without porosity or cracking was obtained. Excellent bonding between the coating and the aluminum alloy substrate was ensured by the strong metallurgical interface. Different ratios of Mo and WC powders were investigated. The chemical composition, microstructure and surface morphology of the clad layer were analyzed using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffractometry (XRD). The surface hardness of the Al specimens clad with Mo/WC is increased seven-fold. The samples have been characterized with abrasive pin-on-disc wear tests. The test results show that the composite coating with a WC hard phase has improved abrasive wear resistance when compared with the as-received Al substrate. The wear mechanism of the specimens is discussed based on microscopic observation of the worn surface.

Cavitation erosion behavior of laser gas nitrided Ti and Ti6Al4V alloy

Abstract A CW 2 kW neodymium-doped yttrium–aluminum–garnet (Nd-YAG) laser was used to carry out the gas nitriding process on commercial pure titanium and Ti6Al4V alloy. The microstructure, surface appearance, hardness depth profile and the cavitation erosion behavior of the nitrided samples in 3.5% NaCl solution were examined. The microstructure of the nitrided layer is mainly composed of TiN dendrites, of which the density gradually decreases towards the interface between the nitrided layer and the substrate. The cavitation erosion resistance of the nitrided samples was significantly improved as compared with the untreated samples. Increase in the surface hardness and the fine dendritic structure of the TiN layer contribute to the significantly enhanced cavitation erosion performance of Ti and Ti6Al4V alloy.

Cavitation erosion and corrosion behaviour of laser surface alloyed MMC of SiC and Si3N4 on Al alloy AA6061

Abstract Laser surface alloying of SiC/Si3N4on AA6061 aluminium alloy was carried out using a 2-kW CW Nd-YAG laser. Different ratios of SiC and Si3N4 powders were mixed and the layers were preplaced by pasting. Subsequent laser surface melting of the layers gave a surface metal matrix composite (MMC). The microstructure, chemical compositions and phase identification of the modified layers were examined using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffractometry (XRD), respectively. The laser surface alloyed MMCs of AA6061-SiC and AA6061-Si3N4 consisted of small amounts of Al4C3/Al4SiC4and AlN, respectively. For specimens alloyed with 100% Si3N4 (AA6061-Si3N4), the cavitation erosion resistance Re was improved by three times as compared to the AA6061 alloy whereas there was no significant improvement in Re of the specimen alloyed with 100% SiC (AA6061-SiC). The surface hardness of the specimens alloyed with SiC/Si3N4 was increased seven times. All the laser alloyed specimens with SiC/Si3N4 showed a decrease in pitting resistance and absence of passivity owing to the aluminium–ceramic interfaces, which favour pit initiation or hindered passivity.

In situ synthesis of TiC reinforced surface MMC on Al6061 by laser surface alloying

Abstract Different weight ratio of titanium and SiC powders were mixed and then laser surface alloyed onto Al6061 substrate aiming at producing a surface metal matrix composite layer. Fine TiC reinforced particles were in situ synthesized during the melting process and a hard MMC layer with good metallurgical bonding to the aluminum substrate was achieved.

Squeeze casting of high-strength aluminium wrought alloy AA7010

Abstract Historically, the 7000 series alloys were developed for high-strength wrought-form applications and are, therefore, unsuitable for conventionally produced castings. However, in this study it is shown that using the squeeze casting process, porosity-free, fine equiaxed grain components of AA7010 composition can be produced successfully. In the first part of this study, the cast structure of aluminium alloy AA7010 solidified under pressure is examined. The results show that the effect of pressure-induced undercooling on the grain structure of the alloy is insignificant. The effect of pressure on the grain structure was detectable only when the melt temperature was well above the liquidus temperature at the time of pressure application. However, a porosity free, fine equiaxed grained structure having an average size of 70 μm was obtained when the alloy was pressurized at a temperature between its liquidus and solidus temperatures. The second part of the study compares the tensile properties of the squeeze cast AA7010 alloy and its wrought product counterpart. The results show that the squeeze cast fully heat-treated alloy exhibits tensile properties between those of the longitudinal and short transverse properties of the wrought material. Unlike the wrought material, in which the grain structure is not fully controllable, squeeze cast material can be produced having isotropic behaviour. It can be argued, therefore, that from this point of view, squeeze castings are better than their wrought counterparts.